Generally, active imaging uses its own light source, conventionally a pulsed laser. By virtue of the directivity of the emission and the energies employed, a signal is received over a great distance, even through a scattering medium. These characteristics have in particular endowed this technology with substantial importance in security and defense. The photons emitted by a laser and reflected by various objects in a theater are collected by a receiver-imager. The joint use of a pulsed laser and a time window on reception allows a section of space containing the targets of interest to be selected.
The field of interest is in particular the pointing of directed laser energy systems requiring correction of effects due to atmospheric turbulence on the free propagation of laser beams; this turbulence interferes with the system and negatively affects performance in terms of accuracy and pointing stability. In addition, tracking a moving target and simultaneously correcting for optical interference introduced by the atmosphere requires substantial control bandwidths (of the order of 1 to 2 kHz) in order to correct for interference characteristically lasting for around 10 ms, and requires covering the widest possible spatial field (of the order of a half-space), while meeting operational constraints such as robustness, autonomy, mobility, deployment time, etc. For applications requiring very high levels of performance (accuracy of the order of microradians, range of several kilometers), this correction entails having access to a measurement of the position of the target with a high angular resolution and a sufficient frequency, in order to ensure the first level of compensation for the deformation of the wave surface (jitter phenomenon). Moreover, the illumination system must operate at a wavelength close, but different, to that of the laser pointing system.
Other examples of applications may be cited, for example the execution of optical communications in free space between a fixed ground station and a flying object in the air or in space, or else a long-distance welding method, which case is frequently encountered in shipyards for assembling large parts and for which the current solutions impose the use of welding stations in contact with or in close proximity to the parts to be assembled.
There currently exists a solution employed in telescopes for correcting for effects due to this atmospheric interference. The case of astronomical instruments has different operational characteristics, however. Specifically, the pointed object is generally point-like and fixed (or moving very slowly), which is generally not the case of the targets envisaged for laser pointing systems. Moreover, both day- and night-time operation is required.
Consequently, there still remains a need for a system for laser-pointing a target that simultaneously meets the aforementioned set of requirements, in terms of accuracy, range and pointing stability, addressable field, bulk and system cost.